1. Field of the Invention
This invention relates to an image forming method in which toner images are formed by developing an electrostatic latent image by, for example, electrophotography, electrostatic recording or electrostatic printing, and to a toner useful for the image forming method.
2. Discussion of the Prior Art
Various electrophotographic image forming methods have been disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications Nos. 42-23910 and 43-24748. The methods typically include the following processes: (a) a surface of a photoconductor is charged (charging process); (b) the charged surface is exposed to light to form an electrostatic latent image thereon (latent image forming process); (c) the latent image is developed with a toner to form a toner image on the photoconductor (developing process); (d) the toner image is transferred directly or indirectly through an intermediate transfer member onto a transfer sheet such as paper (transferring process); and (e) the toner image is fixed to the sheet by application of heat, pressure, solvent vapors, or combination thereof (fixing process).
As the method for developing electrostatic latent images, there are known a wet developing process using a liquid developer containing a pigment or a dye dispersed in an insulating organic liquid medium; and a dry developing process, such as a magnetic brush method (U.S. Pat. No. 2,874,063), a cascade method (U.S. Pat. No. 2,618,552), a powder cloud method (U.S. Pat. No. 2,221,776) and a method using a conductive magnetic toner (U.S. Pat. No. 3,909,258). A toner for use in the dry developing process generally includes a colorant, such as a pigment or a dye, and a binder resin. A magnetic particle such as magnetite is incorporated in the toner for forming a magnetic toner. The toner may be used by itself as a single component developer or may be used in conjunction with a carrier, such as glass beads or iron powder, as a two-component developer.
Toner image fixing methods are broadly classified into two methods, i.e., a contact fixing method and a non-contact fixing method. Typical contact fixing methods include a heating roller fixing method and a heating belt fixing method. Typical non-contact fixing methods include a flash fixing method and an oven fixing method in which a toner image is fixed in a heated atmosphere). Above all, the heating roller fixing method in which a toner image is brought into direct contact with a heating roller is widely used because of its high thermal efficiency and of compactness of the device.
The heating roller fixing method, however, has problems because a large thermal energy is absorbed by the transfer sheet such as paper during the contact of the image-bearing sheet with the heating roller. Thus, when the preset temperature of the heating roller is low, the temperature of the surface of the heating roller is apt to decrease to bring about insufficient fixation of the toner image on the sheet. Such insufficient fixation will not occur when the preset temperature of the heating roller is high. In this case, however, the toner melted upon contact with the heating roller has so low a viscosity that the reproducibility of the fixed toner image is lowered especially in fine line portions thereof. Various toners have been proposed for use in an image forming method utilizing a heating roller fixing method. Japanese Patent No. 2743476 discloses a toner including a polyester resin, and a polar group-containing wax, wherein the melt viscosities of the polyester resin and wax are specifically controlled. Japanese Laid Open Patent Publications No. H03-122661 and No. H04-85550 and Japanese Examined Patent Publication No. H08-16804 disclose a toner including a polyester resin having a specific melt viscosity, and a releasing agent having a specific melt viscosity, wherein the temperature dependency of the melt viscosity of the polyester resin in a temperature range of 80 to 120° C. is specifically controlled. Japanese Laid Open Patent Publication No. H08-12459 discloses an encapsulated toner for fixation to a film including a polyester resin having a specific melt viscosity in a temperature range of 80 to 120° C., and a releasing agent, wherein the temperature dependency of the melt viscosity of the polyester resin is specifically controlled. Japanese Examined Patent Publication No. H07-82250 discloses a toner for fixation to a film including a polyester resin having a specific melt viscosity in a temperature range of 120 to 150° C., an organometallic compound, and a releasing agent, wherein the temperature dependency of the melt viscosity of the polyester resin is specifically controlled. Japanese Examined Patent Publication No. H07-72809 discloses a toner containing a styrene-acrylate copolymer resin having specifically controlled temperature dependency of the melt viscosity thereof. Japanese Laid Open Patent Publication No. H10-246989 proposes a toner containing a specific charge controlling agent and having a specific temperature dependency of the average viscosity. Japanese Laid Open Patent Publication No. H08-220793 discloses a toner having a specific voidage, H08-278659 discloses a toner having a specific particle size distribution and a specific voidage and H10-48874 discloses a toner containing a silicone compound and an inorganic powder and having a specific particle size distribution and a specific voidage.
While these toners have an effect in improving fixation efficiency, an improvement of image quality is not fully satisfactory.
Japanese Laid Open Patent Publication No. H06-230602 proposes a magnetic toner which gives a toner image having a specific ratio of the height thereof before fixation to the height thereof after fixation. The problem to be solved by the Japanese publication is to prevent offset and other troubles during duplex copying and is not concerned with improvement of image quality.
With an increasing demand for high quality images in recent years, particle diameter of toner tends to be made smaller and smaller. With the use of a small particle size toner, a suitable pressure is not easily applied thereto so that the fixation efficiency of the toner image is lowered. This tendency is significant when the heating roller fixation is performed at a low pressure. With a fixing device capable of applying a high pressure to a toner image bearing sheet, satisfactory fixation efficiency is obtainable. However, when the toner image-bearing sheet is relatively thick, the toner image is crushed during fixation to cause deterioration of the image quality. In particular, in the case of digital development, reproducibility of independent dots is adversely affected so that half tone portions of the image are not uniform microscopically. Thus, when the image is observed with naked eyes, human observers are likely to have an impression of a roughness.